Chemical-mechanical polishing (CMP) is used to planarize a semiconductor device. Planarizing the devices reduces problems such as step coverage during subsequent processing. Typically, CMP is performed after the devices are partially formed in the semiconductor material, but before the wafer is diced and the devices are separated one from another.
During CMP, a polishing pad is brought into contact with the front surface of the wafer, where the devices are formed. A polishing slurry is applied between the polishing pad and the wafer, and the wafer and pad are moved relative to each other. The slurry acts upon the wafer to both chemically etch and mechanically wear the devices. The movement between the wafer and pad helps provide a uniform removal of material at the surface of the devices.
As the erosion occurs, the material removed from the wafer tends to clog and mat the surface of the polishing pad, reducing its abrasive action, and causing a reduction in the rate of material removal. Great care must be taken during CMP to ensure that the devices are thinned to within a predetermined range, or they will not function properly.
To ensure uniformity and consistency of device thinning throughout the CMP process, the polishing pad is conditioned by running a rough implement across it. Conditioning is done either intermittently during CMP, or continuously, with the polishing pad rotating across the rough implement in one area, and then across the wafer in another. The rough implement removes the debris that is clogging the polishing pad, and restores the surface of the polishing pad, thus enhancing control of the CMP process. A motion, such as rotation and translation, is imparted to the rough implement so that the polishing pad is uniformly conditioned. The rough implement and the assembly used to hold it are collectively referred to as a conditioner.
A conditioner 10 according to the prior art is depicted in FIG. 1. A grid 20 is used as the rough implement to condition the polishing pad. A grid holder 30 holds the grid 20 in place, and a magnet 40 secures the grid 20 to the grid holder 30. Referring now to FIG. 2, there is shown a mounting bracket 39, which is attached to the grid holder 30. A mechanized arm (not depicted) attaches to the mounting bracket 39, and imparts a rotational movement to the grid holder 30. The rotational movement is conducted through the magnet 40 to the grid 20. The surface 24 of the grid 20 is an abrasive surface, which conditions the polishing pad as the grid 20 rotates across the pad.
It has been discovered that the magnet 40 is unable to hold the grid 20 securely, and slippage occurs at the magnet 40 interfaces between the grid holder 30 and the grid 20. This slippage allows the grid 20 to slow or stop occasionally, reducing the uniformity of the pad conditioning process. Additionally, the slippage between the magnet 40 interfaces erodes material at the interfaces. This material falls onto the polishing pad, and into the polishing slurry, acting as a source of contamination which reduces the controllability of the CMP process, and introduces impurities into the semiconductor devices.
Additionally, the grid 20 can be placed improperly within the grid holder 30, such that the abrasive surface 24 is adjacent the magnet 40. When this occurs, the polishing pad will not be adequately conditioned, and the CMP of the wafer will not be optimized. Further, the abrasive surface 24 will tend to create even more eroded material as slippage occurs between the grid 20 and the magnet 40.
What is needed, therefore, is a method and apparatus of reliably and securely engaging the rough implement used to condition the polishing pad to the apparatus that controls its movement. It is an object of this invention to provide such an apparatus and method.